The present invention relates to a spray drying apparatus for drying liquids containing solid materials in soluble or suspended form.
U.S. Pat. Nos. 3,615,723 and 3,741,273 disclose a spray drying apparatus comprising a drying tower, atomizing devices placed in the upper part of the drying chamber for comminuting the liquid to be dried into droplets, devices for introducing heated drying gas into the upper part of the drying chamber directed towards the droplets, a movable perforated surface placed under the lower end of the drying tower for collecting partially dried particles of solid material, devices for sucking gas together with suspended partially dried solid particles from the drying tower through the movable perforated surface while the particles or a substantial part of the particles settle on the movable perforated surface as an agglomerated porous layer of material, and devices for finish drying the agglomerated, porous layer of material while the perforated surface is moved forwards. The atomizing devices may comprise a number of downwardly directed nozzles for forming liquid droplets which together with a laminar stream of drying gas are conducted down through the chamber. A rotary disc may instead be used for atomizing the liquid. The drying gas may also be used as a turbulent gas stream passing downwards or circulating in the chamber. The partially dried particles will agglomerate and settle as a layer on the movable perforated surface which can be a conveyor belt made of wire netting or woven filter mat.
A drying apparatus of this kind is suitable for drying sensitive and easily decomposable materials, such as foodstuffs and pharmaceuticals since the drying temperature is low.
In order to make such an apparatus operate satisfactorily the layer of particles on the conveyor belt may advantageously be formed as a stable product with an open structure and an even thickness in the transverse direction, which necessitates an even distribution of the particles when settling. This is an aim which is hard to achieve, especially when using large or cylindrical drying chambers, in which case the layer in principle (when using a laminar gas stream) will be thickest in the middle with a decreasing thickness towards the edge of the conveyor belt, even if the belt covers the entire bottom of the chamber. Even if several atomizing nozzles are uniformly distributed in the upper part of the drying tower so that the partially dried particles get uniformly distributed in the drying gas and the suspension forms a laminar downward stream, the thickness of the settled layer will vary anyway.
If a rotary disc placed in the upper central part of the drying tower is used as atomizing device the particles will be hurled towards the walls of the chamber, and thus a thicker layer will be formed in the outer zones of the conveyor belt than in the central zone, and formation of an agglomerated uniform powder structure will not be achieved.
In order to overcome this disadvantage and obtain a layer of particles of uniform thickness on the conveyor belt, it has been proposed to provide a turbulent bustle zone between the chamber and the conveyor belt, cf. (DK patent application No. 1583/88 (Mar. 23, 1988)). This solution more or less solves the problem of creating a layer of settled particles with an even thickness in the transverse direction on the conveyor belt. It is, however, disadvantageous that the particles, due to a relatively high gas velocity and because of introduced supplementary process air, settle as a compact layer which will cause great resistance to gas penetration, result in decreased drying velocity and increase the risk of blocking the mat and the supporting movable perforated surface.